Radio frequency (RF) devices such as cellular telephones generally include RF switching circuitry for directing the flow of RF signals within the device. The RF switching circuitry may be comprised of one or more RF switching elements arranged in a series or shunt configuration. Each one of the switching elements may be adapted to maintain either an ON state or an OFF state, depending on a control signal applied to the switching element. Accordingly, RF switching circuitry often includes switching support circuitry in order to maintain the switching elements in a desired state.
FIG. 1 shows an example of conventional RF switching circuitry 10. The conventional RF switching circuitry 10 includes a low-dropout voltage regulator 12, biasing circuitry 14, input circuitry 16, switch driver circuitry 18, and one or more RF switching elements 20. The low-dropout voltage regulator 12 is coupled to the biasing circuitry 14, the input circuitry 16, and the switch driver circuitry 18. An output of the biasing circuitry 14 is coupled to the switch driver circuitry 18. The input circuitry includes one or more input control terminals 22 and an output coupled to the switch driver circuitry 18. The output of the switch driver circuitry is coupled to the RF switching elements 20. Each one of the low-dropout voltage regulator 12, the biasing circuitry 14, the input circuitry 16, the switch driver circuitry 18, and the RF switching elements 20 are coupled to a common ground 24.
In operation, the low-dropout voltage regulator 12 receives a battery voltage V_BATT and generates a regulated supply voltage V_SUPP. The biasing circuitry 14 receives the regulated supply voltage V_SUPP from the low-dropout voltage regulator 12 and generates a biasing signal V_BIAS for maintaining the RF switching elements 20 in either an ON state or an OFF state. The input circuitry 16 receives one or more input control signals V_INC from the input control terminals 22, and processes the input control signals V_INC to generate one or more processed input control signals V_INP. The switch driver circuitry 18 uses one or more of the processed input control signals V_INP from the input circuitry 16 together with the biasing signal V_BIAS from the biasing circuitry 14 and the supply voltage V_SUPP from the low-dropout voltage regulator 12 to maintain each one of the RF switching elements 20 in either an ON state or an OFF state. Each one of the processed input control signals V_INP controls the state of one of the RF switching elements 20. Based on the state of each one of the RF switching elements 20, an RF signal presented at an input terminal 26 of the conventional RF switching circuitry 10 is selectively passed to an output terminal 28 of the conventional RF switching circuitry 10.
Although effective at directing the flow of RF signals, the conventional RF switching circuitry 10 may introduce spurious noise into RF signals passing through each switching element in the RF switching elements 20, as will be discussed in further detail below. Accordingly, the performance of a device incorporating the conventional RF switching circuitry 10 will suffer.
FIGS. 2A and 2B show exemplary configurations of the RF switching elements 20 shown in FIG. 1. Specifically, FIG. 2A shows four RF switching elements RF_SW coupled in series between an input terminal 26 and ground. According to one embodiment, each one of the RF switching elements RF_SW is a metal-oxide-semiconductor field-effect transistor (MOSFET) however; any type of switching device including a field-effect transistor (FET), a metal semiconductor field-effect transistor (MESFET), a bipolar junction transmitter (BJT), or the like may be used without departing from the principles of the present disclosure. Although the control terminals of each one of the RF switching elements RF_SW are shown coupled together into a single control terminal 30, the control terminals of each RF switching device RF_SW may also be individually controlled. Additional circuitry such as multiplexers and de-multiplexers (not shown) may be used to relay the control information for each RF switching device RF_SW between the input circuitry 16 and the RF switching elements 20. FIG. 2B shows four RF switching elements RF_SW coupled in series between an input terminal 26 and an output terminal 28. By coupling the RF switching elements RF_SW in series, an RF signal present at the input terminal 26 is distributed evenly across the elements, thereby allowing the RF switching elements 20 to effectively direct high amplitude RF signals.
FIG. 3 shows an example of the conventional biasing circuitry 14 shown in FIG. 1. For context, the low-dropout voltage regulator 12 is also shown. The conventional biasing circuitry 14 is a negative charge pump including an oscillator 32, charge pump switching circuitry 34, and an output capacitor 36. The low-dropout voltage regulator 12 is coupled to the oscillator 32 and the charge pump switching circuitry 34. An output of the oscillator 32 is coupled to the charge pump switching circuitry 34. An output of the charge pump switching circuitry 34 is coupled to the output capacitor 36, which is coupled between the output of the charge pump switching circuitry 34 and ground.
In operation, the oscillator 32 receives the supply voltage V_SUPP from the low-dropout voltage regulator 12 and generates an oscillating signal V_OSC. The oscillating signal V_OSC is delivered to the charge pump switching circuitry 34, where it is used to generate a stepped-up output voltage V_SU from the supply voltage V_SUPP. The stepped-up output voltage V_SU is then filtered by the output capacitor 36 to generate the biasing signal V_BIAS.
Although effective at generating a biasing signal V_BIAS for maintaining the RF switching elements 20 in either an ON state or an OFF state, the conventional biasing circuitry 14 shown in FIG. 3 is slow to produce the bias signal V_BIAS, and thus cannot quickly transition the RF switching elements 20 between states. Due to the nature of modern RF communications standards, fast switching of RF signals is becoming increasingly essential. Accordingly, biasing circuitry for an RF switching element is needed that is capable of quickly transitioning an RF switching element between an ON state and an OFF state, while minimizing noise introduced into a passing RF signal.
FIG. 4 shows details of the charge pump switching circuitry 34 shown in FIG. 3. For context, the low-dropout voltage regulator 12, the oscillator 32, and the output capacitor 36 are also shown. The charge pump switching circuitry 34 includes a flying capacitor C_FLY including a positive terminal 38 and a negative terminal 40, a first switch SW_1 coupled between the low-dropout voltage regulator 12 and the positive terminal 38 of the flying capacitor C_FLY, a second switch SW_2 coupled between the positive terminal 38 of the flying capacitor and the output capacitor 36, a third switch SW_3 coupled between ground and the negative terminal 40 of the flying capacitor C_FLY, and a fourth switch SW_4 coupled between the negative terminal 40 of the flying capacitor C_FLY and ground (referred to collectively as the switches SW). Each one of the switches SW is coupled to the oscillator 32, such that the oscillating signal V_OSC determines when each switch is in the ON state or the OFF state. Notably, the oscillating signal V_OSC supplied to the second switch SW_2 and the third switch SW_3 is inverted, such that when the first switch SW_1 and the fourth switch SW_4 are in an ON, or closed state, the second switch SW_2 and the third switch SW_3 are in an OFF, or open state, and vice versa. Accordingly, during a first charging cycle, the first switch SW_1 and the fourth switch SW_4 are closed, while the second switch SW_2 and the third switch SW_3 are open, thereby charging the flying capacitor C_FLY to the supply voltage V_SUPP. During a second discharging cycle, the second switch SW_2 and the third switch SW_3 are closed, while the first switch SW_1 and the fourth switch SW_4 are open, thereby discharging the flying capacitor C_FLY across the output capacitor 36. The charging cycle and the discharging cycle are repeated in order to generate the biasing signal V_BIAS.
FIG. 5 shows an additional example of the conventional biasing circuitry 14 shown in FIG. 1. For context, the low-dropout voltage regulator 12 is also shown. The conventional biasing circuitry 14 is a negative charge pump including an oscillator 42, charge pump switching circuitry 44, an output capacitor 46, a voltage detector 48, frequency selection circuitry 50, and one or more frequency dividers 52A-52N (referred to collectively as the frequency dividers 52). In operation, the oscillator 42 receives the regulated supply voltage V_SUPP from the low-dropout voltage regulator 12 and produces a high-frequency oscillating signal V_OSC_HF. Depending on the mode of operation of the conventional biasing circuitry 14, the high frequency oscillating signal V_OSC_HF is either delivered to the charge pump switching circuitry 44 directly, or delivered to the charge pump switching circuitry 44 through the frequency dividers 52. In response to the high frequency oscillating signal V_OSC_HF or the frequency divided high frequency oscillating signal, the charge pump switching circuitry 44 produces a stepped-up output voltage V_SU, as described above. The stepped-up output voltage V_SU is filtered by the output capacitor 46 to produce the biasing signal V_BIAS. The voltage detector 48 senses the output of the charge pump switching circuitry 44 and makes adjustments to the frequency selection circuitry 50 in order to maintain the biasing signal V_BIAS at a desired level.
In a “boost” mode of operation, the high frequency oscillating signal V_OSC_HF is delivered directly to the charge pump switching circuitry 44. In response to the high frequency oscillating signal V_OSC_HF, the conventional biasing circuitry 14 quickly produces a biasing signal V_BIAS. By quickly producing the biasing signal V_BIAS, the state of the RF switching elements 20 may be changed faster than would otherwise be possible. In a normal mode of operation, the high frequency oscillating signal V_OSC_HF is delivered to the charge pump switching circuitry 44 through the frequency dividers 52. Accordingly, the frequency of the signal is reduced, resulting in a slower production of the biasing signal V_BIAS. The conventional biasing circuitry 14 may use the “boost” mode of operation to quickly change the state of the RF switching elements 20, and use the normal mode of operation to maintain the state of the RF switching elements 20. Control circuitry may be coupled to the frequency selection circuitry 50 in order to switch the conventional biasing circuitry 14 between the “boost” mode of operation and the normal mode of operation.
Although effective at quickly changing and maintaining the state of the RF switching elements 20, the conventional biasing circuitry 14 shown in FIG. 5 generates an excessive amount of spurious noise at high frequencies due to the constant generation of the high frequency oscillating output signal V_OSC_HF. The high frequency noise may couple with the RF switching elements 20 and distort passing RF signals, thereby degrading the performance of a device in which the conventional biasing circuitry 14 is incorporated. Accordingly, there is a need for circuitry that is capable of quickly changing the state of one or more RF switching elements while reducing or eliminating noise coupled to the RF switching elements.